Historically, shoe bottoms have been constructed for the most part with flat top and bottom surfaces. This sort of shoe bottom was normally made of single density polyurethane (PU) or blown polyvinylchloride (PVC) materials. The upper of the shoe would be glued onto the top of the sole or the upper could be "direct attached" through a molding process that would capture the upper in the molded sole. The bottom could be the lowermost layer of the sole if the urethane was sufficiently abrasion-resistant. Alternatively, a rubber outsole could be cemented onto the unit bottom, as is typically done in the manufacture of running shoes.
Eventually, it became known to contour the top surface of the bottom unit to provide a heel cup and a slight arch. This made the shoe more comfortable because the foot would rest on a surface similar to its shape as opposed to a flat surface which felt like flat feet on a firm floor.
When the contour surface is used with a dual-density bottom, that is, two different densities of PU, the lowermost (outer) portion is formed of a uniform thickness. This portion can be used for its abrasion resistance. The softer portion is positioned on top of this uniform portion to provide comfort and cushioning, since the firmer material would be too hard for comfort. Further, the respective volumes of the softer and firmer materials are such that the volume of soft material is maximized and the volume of firmer material is minimized.
The prior known structures have always had to trade cushioning for stability. If the bottom is soft for good cushioning, then the foot rocks from side to side, which is unstable. Even existing soles with contoured topmost surfaces have this type of trade-off. It has been proposed, for example, in U.S. Pat. Nos. 4,399,620 (Funck) and 4,446,633 (Scheinhaus et al.) to contour the lower wear-resistant layer but to provide a relatively flat second layer that is deformable rather than double contoured. The designs taught in each of these patents, however, provides a flat surface that must be deformed by the foot to obtain a satisfactory shape, thus losing much of the support which was to be provided by the bottom.